This invention has to do with dual component developers for use in electrophotographic recordings and is especially concerned with coated carriers used with such developers.
Developer materials comprise a mixture of toner and carrier particles with the size of the toner particles being relatively small when compared to the size of the carrier particles. Each of the toner and developer have opposite triboelectric charges so that the toner particles are held to the surface of the carrier particles until the developer is moved into contact with a latent electrostatic image at which point the toner particle separate from the carrier particles and attach themselves to the latent image.
The developer material is then re-cycled so as to pick up more toner particles in order to repeat the electrophotographic process. Constant cycling of the carrier particles can cause the particles to lose their configuration, as well as most coating materials used, reduce their cross-sectional dimensions and result in impaction of the toner materials which changes the value of the carrier's triboelectric charge.
The loss or change in the carrier's triboelectric charge has dramatic effects on the quality of the copy produced by the electrophotographic process.
Dual component developers with coated carriers have proven useful and a good description of the problems associated with the carrier components may be had by referral to U.S. Pat. No. 3,778,262 issued to International Business Machines Corporation and disclosing Teflon-S coated carrier components.
The advantages of coated carriers are known, the most important of which to prolong the useful life of the carrier by improving the wear resistance of the carriers and by maintaining the proper triboelectric change on the carrier. Coatings on carriers further prevent oxidation of carriers comprised of ferromagnetic substrate materials. Coating materials have been used with varying results. Lacquer coated carriers, resin coated steel beads along with other resin coatings containing styrene-acrylic co-polymers, vinyl chloride-acetate co-polymers, cellulose acetate buyrate and nitrocellulose composition containing charge orienting dyes are known in the industry.
A well known method for applying the coating of the present invention is the WURSTER described in U.S. Pat. Nos. 2,648,609, 2,799,241; and 3,196,827 and 3,241,520 to WURSTER et al. An example of the WURSTER process used to coat flowable carriers is exemplified by U.S. Pat. No. 3,798,167.
It is an object of the present invention to provide a developer with a carrier having a high wear resistance.
It is a further object of the present invention to provide a developer with a carrier having a stable triboelectric performance over its lifetime.
It is a further object of the present invention to provide a dual component carrier with low toner impaction.
It is a further object of the present invention to provide a carrier coating that will enable the use of substrates heretofore not desired in developers.
It is a further object of the present invention to provide carriers having a longer lifetime of use in dual component toners.